If you sew, you could design and make one (400d pack cloth would probably be my choice), but you'd have to use either a waterproof zipper or make a dry bag style closure at the large end. And spend plenty of time sealing all the seams.

I once made a guitar case cover out of coated Cordura, and it took some time, but wasn't awfully difficult. I had a Gibson Super 400 at the time, but didn't have the leatherette cover that went over the case. I didn't try to waterproof it, though.

Another possibility is to find a smallish cloth bag, maybe for a solid body guitar, add some padding, and use a large plastic bag for waterproofing.

These guys say, "We will make dry bags for just about anything."

https://www.jpwinc.com

And there may be others. I imagine having one made might be expensive, but I'm not sure.